This invention relates to stabilizing signals from a radioactive well logging tool in which data pulses generated responsive to radiation impinging on the scintillation crystal from a submerged formation are analyzed to determine both the count rate and the energy content of the radiation being detected.
It is well known that in oil and gas wells, physical characteristics of the formations surrounding the well and the chemical content of formations and fluids in the formations can be determined from radiation emanating from the formation. The radiation detected may be either radiation naturally originating in the formation, or may be induced radiation caused by irradiating the formation during a well logging operation.
It is also well known that radiation from a submerged formation may be detected by use of a scintillation crystal which gives off light pulses proportional to the energy of the radiation absorbed by the crystal. The light pulses are then detected by a light detector which typically produces electrical pulses which are proportional to the intensity of the light pulses detected. The electrical pulses are then transmitted to the surface over a transmission system where the number and height of the electrical pulses are analyzed to determine the characteristics of the formations and the fluids contained therein.
The electrical pulses received for analysis at the surface will be affected by errors introduced by the light detection system and the transmission system. In the past, reference pulses either naturally appearing in or introduced into the electrical pulse spectra have been used to correct the electrical pulses before they are analyzed. For instance, in U.S. Pat. No. 3,829,686 a particular known energy peak occurring in the spectra of the radiation detected from the subsurface formation is used as a reference. The peak chosen in this instance is the 2.22 MEV hydrogen gamma ray energy spectrum peak.
In U.S. Pat. No. 3,916,685 the reference pulses are generated by an oscillator in the logging tool and impressed on the transmission system with the electrical pulses produced by the light detector. However, since the reference pulses are produced independent of the light detector, the generated reference pulses will not include errors caused by the light detector itself. Also in U.S. Pat. No. 3,916,685, it is suggested that the scintillation crystal may be doped with an alpha emitting isotope to provide to the light detector reference light pulses to give the desired reference electrical pulses.
It has been found when the scintillation crystal itself is doped with an alpha emitter, that the resulting light pulses are attenuated as they travel through the scintillation crystal. This effect smears the energy content information of the data signal reference pulses such that a distinct peak of reference pulses is not formed.
The present invention provides a scintillation crystal radiation detector for use in oil well logging having a main scintillation crystal and a light detector for detecting scintillations in the crystal due to radiation from submerged formations, and includes a reference crystal embedded in the main crystal for providing reference light pulses to the light detector. The reference crystal is doped with a reference source of mono-energetic radiation, and is placed in the main crystal such that scintillations in the reference crystal are detected by the light detector directly and separately from the scintillation in the main crystal.
The preferred light detector of the present invention is a photomultiplier tube which provides a data signal having electrical pulses proportional to the intensity of light flashes in the main crystal and the reference crystal.
The electrical pulses are transmitted to the surface where the surface equipment, including an electronic circuit which detects any shift in the peak of the reference pulses, provides a feedback signal which adjusts the gain of an amplifier for correcting drift in the data signal responsive to drift in the reference signal.
In the particular embodiment illustrated herein, single channel analyzers are used to detect and record that portion of the spectrum indicative of hydrogen and chlorine. The energy of the reference source is chosen to be in a part of the spectrum which will not be masked by background radiation from the subsurface formations; the preferred range being from about 4.5 to about 6.5 MEV.
The surface equipment additionally includes electronic circuits to select and display preselected portions of the spectrum, and an electronic circuit which subtracts from the corrected spectrum the known value of the reference pulses.
Thus, this invention provides a well logging tool having a clearly delineated reference peak in the transmitted data signal which allows the data signal to be corrected for drift in the light detector, the supply voltage to the light detector, and the data transmission system. The reference signal is provided by a source of mono-energetic radiation in a reference crystal which is embedded in the main crystal of a scintillation crystal detector. The gamma equivalent energy and count rate of the reference crystal are selected to produce a reference peak in the data signal spectrum such that the peak is not masked by background pulses and the reference crystal is arranged in the main crystal such that the reference pulses are unattenuated by the main crystal.